1. Field of the Invention
The present invention is directed to an implantable heart defibrillator of the type having a pulse generator, controlled by a control unit, for emitting defibrillation pulses.
2. Description of the Prior Art
Due to a high intracellular concentration of potassium, a heart tissue cell "at rest" is in a polarized condition, i.e., its interior is more negative than its exterior. When stimulated (excited) by the presence of an electrical field, the cell depolarizes so that its interior is momentarily more positive than its exterior. When the electrical field is removed, the cell requires a certain amount of time to repolarize. When the cell is in this condition, it is said to be refractory. A refractory cell will not respond to a renewed stimulation. Both healthy and pathological depolarization waves are conducted in a heart by the successive depolarization and repolarization of cells along a propagation path. A stimulated cell, in turn, causes neighboring cells to be similarly stimulated, thereby propagating the depolarization wave.
If the heart's natural pacemaker is unable to initiate such depolarization waves, it can be artificially assisted by pacemaker-generated stimulation pulses. The reverse situation applies in the case of fibrillation, i.e., the disorganized generation of depolarization waves within the heart. For fibrillation therapy, i.e. defibrillation, the goal is to prevent the propagation of the disorganized depolarization waves. This is accomplished in conventional defibrillation by applying a sufficiently strong electric field to the entire heart, or substantial portions thereof, so as to make as many heart cells as possible refractory for an unnaturally long time. Since the refractory cells are incapable of being stimulated, this prevents the disorganized depolarization waves from being further propagated, and the heart's natural pacemaker, or an artificial pacemaker, thereafter regains control of the generation of the depolarization waves.
It is known that the heart cells are anisotropic, i.e., an electrical field, such as that generated by a defibrillation pulse, will stimulate a heart tissue cell more readily if the cell has a given (preferred) orientation in relation to the defibrillating electrical field. The electrical field strength required to stimulate a heart tissue cell can vary between two orthogonal directions by a factor of 2 to 5. Therefore, any specified area of heart tissue will contain a collection of differently oriented cells, some of which will be susceptible to stimulation with a lower magnitude electrical field, while others will require a higher magnitude field in order to be stimulated. In conventional defibrillating techniques, the defibrillation pulse which is generated is of such a high magnitude as to ensure that all tissue cells, regardless of their orientation, will be stimulated (depolarized). This requires the generation of a defibrillation pulse at a voltage on the order of 500 to 1000 volts, and having an energy content of 10 joules or more.
A cardioversion system is disclosed in German OS 3715822 wherein the cardioversion pulse is chopped into a sequence of brief pulses in order to save energy. A normal, single cardioversion pulse will have an exponentially decaying waveform or envelope. The general waveform or envelope of the pulse "package" consisting of the sequence of brief pulses in this known system still displays the convention exponentially decaying configuration, i.e., the amplitude of the individual pulses in the pulse package produced by chopping is the same as that of a conventional cardioversion pulse.